Preheat oven to 325 degrees F. Grease a 9-by-13-inch baking pan. Steam the cauliflower by placing in a saucepan in 1/2-inch of boiling water, cover with lid and cook for 5 minutes. Drain. Place cooked cauliflower in a bowl and mash using a potato masher. Remove the stems and scoop out the caps of mushrooms. Chop stems for later use. Combine yogurt, cheddar cheese, Parmesan cheese, bran flakes, bell peppers, salt, pepper and garlic powder in a medium bowl. Stir in cauliflower and 1/2 cup of the reserved chopped mushroom stems. Stuff the cauliflower mixture into the hollowed mushroom caps. Sprinkle with paprika. Bake, uncovered for 20 minutes.